The Direct AC driven LED light is probably the most low cost of the traditional LED lamp architectures, due to fewer components, easy configuration and no electromagnetic interference (EMI). However, conventional Direct AC driven LED lighting usually suffers from low efficiency, low frequency flicker and low power factor.
With reference to FIG. 1, FIG. 1 illustrates the conventional current source driven LED light that drives LED diodes directly from a rectified AC voltage. The current source 2 in this circuit architecture of FIG. 1 is also known as the Current Regulating Device (CRD), and such a circuit architecture may suffer a large inefficiency when large voltages appear across the CRD. This situation occurs when the rectified AC voltage is higher than the cumulative voltage of the diodes of the LED string 3. If the rectified AC voltage is lower than the cumulative voltage of the diodes then the circuit will suffer from low frequency flicker. In that situation the LED string 3 turns off once every half cycle leading to a flicker frequency of 100 and 120 Hz for AC input voltages, VAC, of 50 and 60 Hz respectively.
Many designs add a large capacitor Cf after the rectifier 1 to change the pulsating waveform from the rectifier to a waveform more closely resembling a DC voltage. The remaining ripple seen after the rectifier 1 is a function of the size of the added capacitor Cf and the magnitude of the load (i.e., LED string 3). As the ripple decreases, the current source 2 that drives the LEDs can become efficient. However, even if the capacitor Cf is made so large as to create an ideal DC voltage after the rectifier, there are still problems with efficiency. Namely, the number of the LED diodes in the string must be designed so that there will always be sufficient voltage across the string to keep them all lit. The variation in LED voltage and input AC voltage VAC require using fewer LED diodes than an ideal number. That means that the rectified voltage will always be higher than the sum voltage of the diodes of the string. Any extra voltage across the current source 2 represents wasted power.
Therefore, there is a need for an approach to provide a device or means so that the “ON” voltage of the LED string is closely matched to the rectified voltage at any given moment.